Multiple compound conductor current-limiting device

ABSTRACT

A current limiting device including at least one superconductor and at least one nonsuperconducting shunt resistor in parallel with each superconductor. Each superconductor has a first main superconductor face in contact with a main shunt resistor face of a shunt resistor so as to form a compound conductor generally in the form of a meandering band having a band width greater than approximately 3.5 times the superconductor thickness. At least one insulator has opposing faces each in contact with a compound conductor. Multiple compound conductors are arranged such that current flows in opposite directions through adjacent compound conductors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is based on a current limiter device.

2. Discussion of Background

Reference is made to U.S. Pat. No. 4,961,066, in which a current limiteris disclosed, for fast current limitation in the event of shortcircuits. The current limiter can include a support insulator which isrod shaped, tubular or a planar layer structure, in each case consistingof a support insulator. A thin superconducting layer is appliedextensively thereon and a normal conductor resistive layer is appliedextensively on the latter. The last two layers can be repeated one afterthe other. In this case, the resistance of the non-superconductingresistor is less than that of the superconductor in the normalconducting state. Disadvantages are large power losses in AC operationas well as relatively long conductors.

In regard to the relevant prior art, reference is also made toEP-A1-0,406,636. The latter provides, for limiting overcurrents in anelectrical line of an AC circuit, for example as a result of a shortcircuit, a current limiter in which an induction coil is connected in abranch in parallel with a high temperature superconductor. Thesuperconductor is arranged inside the induction coil and is alsoconnected in parallel with a non-superconducting shunt resistor.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to provide a furtherdevelopment of a current limiter device of the type mentioned at theoutset in such a way that it is suitable for the resistive limitation ofboth direct and alternating currents. The intention is to limit anovercurrent, in the event of a short circuit, to a predeterminablemultiple of the rated current.

An advantage of the invention consists in that the current limiterdevice is simple and compact. The superconducting part of the currentlimiter is of modular design, that is to say that the superconductor issubdivided into units which, should the need arise, can be removed andreplaced separately.

According to an advantageous configuration of the invention, thecompound conductors, consisting of superconductors and normalconductors, used in the current limiter can be arranged with lowinductance. By virtue of a suitable conductor arrangement, AC losseswhich occur in the case of AC applications can be greatly reduced.

The current limiter device can also be used as an active switchingelement by introducing it into an external magnetic field. In this case,use is made of the fact that the critical current is very greatlyreduced in magnetic fields. By switching on the external magnetic field,it is therefore possible to reduce the current in the superconductor toa fraction of the rated current.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows, in cross section, a modular arrangement of currentlimiters in a magnetic field coil,

FIG. 2 shows, in cross section, a current limiter according to FIG. 1,

FIG. 3 shows, in a cross section, a superconductor of the currentlimiter according to FIG. 2, with a meandering conductor track,

FIG. 4 shows signal diagrams of AC losses with current limiters, and

FIGS. 5-8 show current limiters with different layer sequences.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, FIG. 1shows 4 current limiters or current limiter modules (5), which arearranged parallel to one another in a cryostat (7) filled with liquidnitrogen, are electrically connected in series and are connected to anelectrical line (6). During operation, a current (I) which, in the eventof an overcurrent, for example as a result of a short circuit, isintended to be limited by the current limiter modules (5) to 3 times-5times a predeterminable rated current (I_(R)) which flows through theelectrical line (6). The cryostat (7) is arranged within a magneticfield coil (8).

FIG. 2 shows the layer structure of a current limiter module (5)according to FIG. 1. A thin silver buffer layer (2) whose thickness isin the range of 1 μm to 5 μm is in each case applied onto a first mainface (1a) and onto a second main face (1b), opposite the former, of aplate-shaped ceramic sheet or of an insulator (1) having a thickness(d₁). Suitable materials for the insulator (1) are insulators,obtainable in sheet form, which have sufficient thermal stability andundergo, between room temperature and 77 K., a long-term thermalalteration which is comparable with that of the superconductor (3, 3').Glass fiber reinforced cast resin sheets or MgO ceramic sheets arepreferably used. They are simply applied to the buffer layer (2) using acommercially available adhesive. This step is required forsuperconductor sheets (3, 3') which have been cast in a silver mold thatis removed after casting, but is not required when using cast substrateson a nickel based alloy or ceramic which need not be removed afterproduction of the superconductor (3, 3') and can be used formechanically stabilizing the latter.

Plate-shaped high temperature superconductors or superconductors (3,3'). having rectangular cross sections and meandering configurationsaccording to FIG. 3 and respective thicknesses (d_(SC)) are applied ontothe two buffer layers (2), in extensive contact with the latter. In thiscase, a first main face (3) of each superconductor (3, 3') is, in orderto electrically stabilize it, in good electrically. conductive contactwith the respective buffer layer (2). The two superconductors (3, 3')have a mutual layer separation or conductor separation (Δ).

A second main face (3b) of the respective superconductor (3, 3') is, inorder to electrically and thermally stabilize it, in extensive goodelectrically conductive contact with a first main face (4a) of arespective non-superconducting resistor or normal conductor (4, 4')having a thickness (d_(NC)). A second main face of the normal conductor(4, 4'), opposite the first main face (4a), is designated (4b). Suitablemetals for the normal conductors (4, 4') are, in particular, those whichhave a resistivity at room temperature of >10 μΩ cm and are stillductile at -200° C. Preferable ones are: tin, zinc, bismuth and alloysthereof as well as non-magnetic metals based on steel or nickel. Thenormal conductors (4, 4') can be applied onto the superconductor (3, 3')by electrolytic methods, flame spraying, plasma spraying, bonding usinga conductive adhesive, soldering or sintering a cold-sprayed metalpowder. The thickness of the normal conductor layer (4, 4') should besuch that the electrical resistance of this layer is approximately equalto that of the adjoining superconductor layer (3, 3') in thenon-superconducting state, for example 50 μm when the thickness (d_(SC))of the superconductor (3, 3') is 1 mm. The contact resistivity,expressed in terms of area, between the second main face (3b) of thesuperconductors (3, 3') and the first main face (4a) of the normalconductors (4, 4') should be <1 mΩ cm², preferably ≦10 μΩ cm².

FIG. 3 shows a cross section through the superconductor (3) according toFIG. 2, in a section that is perpendicular to the layer representationin FIG. 2. A band-shaped conductor is produced using cuts or meandergaps (9) in a sheet with a rectangular, preferably square contour.Silver contacts (10, 11) are applied to this conductor at the end, forelectrically connecting it. Neighboring meander gaps (9) have aperpendicular separation (b). corresponding to a meander path width. Themeanders are most simply produced by reciprocal cuts by milling orsawing or laser or water-jet cutting, and this can be done beforeapplying the normal conductor (4, 4') or else before applying themechanical stabilization, i.e., is to say the insulator (1).

The meandering superconductors (3, 3') are arranged, in the currentlimiter modules (5), on both sides of the insulator (1) in such a waythat the current (I) flows in the opposite direction in respectivelyopposite meander paths, so that the self-field components perpendicularto the band plane compensate for each other. The effect of this is thatthe current limiter modules (5) are low-inductance and low-loss.

FIG. 4 shows, in AC loss curves (12) and (13) for different values ofthe ratio of the meander path width (b) to the thickness (d_(SC)) of thesuperconductor (3, 3'), the AC losses which occur in the abovementionedcurrent return line. In this case the conductor separation (Δ) in mm isplotted on the abscissa and the ratio of the electrical AC power loss(P) to a length (l) of the superconductor (3, 3') in mW/m is plotted onthe ordinate. The AC loss curve (12) shows that, for b/d_(SC) =2, the AClosses decrease with increasing conductor separation (Δ). In comparison,the AC loss curve (13) shows that while with b/d_(SC) =15 the AC lossesincrease with increasing conductor separation (Δ). The current returnarrangement in a current limiter module (5) only leads to a reduction inthe losses with increasing conductor separation (Δ) when the meandertrack of the superconductor (3, 3') is sufficiently flat, that is to saywhen the ratio b/d_(SC) is sufficiently large, up to a critical value of3.5. For bands with b<3.5·d_(SC), the current return increases the ACpower loss (P) instead of reducing it.

FIGS. 5-8 show a simplified representation of various layer sequenceswhich can be used instead of the layer structure in FIG. 2. In this case(NC) designates normal conductors (4, 4', 14, 15) and (SC) designatessuperconductors (3, 3').

According to FIG. 5, a compound conductor or laminate consisting of anormal conductor (4, 4') and a superconductor (3, 3') can be extensivelyconnected to the insulator (1) or the respective buffer layer (2) insuch a way that the normal conductor (4, 4') is in each case arranged onthe insulator side.

According to FIG. 6, a compound conductor consisting of a normalconductor (4, 4') and a superconductor (3, 3') can be extensivelyconnected to the insulator (1) or the respective buffer layer (2) insuch a way that the insulator (1) is in superficial contact, via one ofits main faces, with the superconductor (3), according to thearrangement of FIG. 2, and via its other main face with a normalconductor (4'), according to the arrangement of FIG. 5. It is alsopossible to provide a further insulator (1') which is extensivelyconnected to the superconductor (3') via a buffer layer (2).

FIG. 7 shows a layer structure according to FIG. 5, in which the outerfaces of the superconductors (3, 3') are in extensive, highly conductiveelectrical contact with additional normal conductors (14) and (15).

FIG. 8 shows a current limiter module (5) which has a layer structureaccording to FIG. 7 on one side of the insulator (1) and a layerstructure according to FIG. 2 on the other side.

In the current limiter modules (5) according to FIG. 1, thesuperconductors (3, 3') are resistively coupled into an electricalcircuit. Below a certain critical current strength (j_(c)) thesuperconductor (3, 3') is in the superconducting state and therefore hasvirtually no electrical resistance. If the critical current strength isexceeded, for example because of a short circuit, then thesuperconductor (3, 3') undergoes transition into its normal conductingstate. The resistance which results therefrom limits the current to avalue which is much smaller than the short circuit current.

Important factors are the dimensioning of the superconductor (3, 3'),its electrical, thermal and mechanical stabilization, the AC losseswhich result during operation and the connections between the currentlimiter modules (5).

The electrical and thermal stabilization is achieved using at least onenormal conductor (4, 4', 14, 15) as a parallel conductor, which mustlocally be in good electrical and thermal contact with thesuperconductor (3, 3'). This bypass resistor (4, 4', 14, 15) can, shouldneed be, locally take a part of the current from the superconductor (3,3') and thereby protect the latter from excessive heating anddestruction. In order for it to be possible to relieve the load on thesuperconductor (3, 3') effectively, the bypass resistance (4, 4', 14,15) should not be greater than the normal resistance of thesuperconductor (3, 3'). The thickness (d_(NC)) of the bypass resistor(4, 4', 14, 15) must consequently be ≧d_(SC) ρ_(NC) ·ρ_(SC), ρ_(NC) andρ_(SC) being the resistivities of the bypass resistor (4, 4', 14, 15)and of the superconductor (3, 3'), respectively. Since the intention isfor the bypass resistor (4, 4', 14, 15) to receive as much heat aspossible, a high thermal inertia and consequently a high resistivityρ_(NC) are beneficial.

In the operating state, the superconductor (3, 3') must be able to carrythe rated current (I_(R)), which gives a lower limit for itscross-sectional area F, according to:

    F≧1.414·I.sub.R /j.sub.c.

In the limiter state the current (I) is intended to rise to at most ntimes the rated current (I_(R)), values of between 3 and 5 beingrequired in practice. This requirement gives the minimum conductorlength (l) of the superconductor (3, 3'), according to:

    l≧b·(d.sub.SC /ρ.sub.SC +d.sub.NC /ρ.sub.NC)·1.414·U.sub.R /(ln·I.sub.R),

U_(R) being the rated voltage of a current source (not represented) andb being the strip width of a compound conductor consisting of a bypassresistor (4, 4', 14, 15) and a superconductor (3, 3').

The AC power loss (P) of a superconductor (3, 3') through which currentflows depends greatly on the local magnetic field (self-field andpossible external fields). In the case of band-shaped superconductors(3, 3'), as they are used according to FIG. 1, above all the fieldcomponents which are perpendicular to the band plane have a veryunfavorable effect on the AC power loss (P). The conductor geometry musttherefore be realized in such a way that the field in the superconductor(3, 3') is oriented predominantly parallel to the band plane. In asingle thin current-carrying band, the magnetic field in the conductoris for the most part perpendicular to the band plane, in which case theAC power loss (P) would not be acceptable for the application. Anefficient reduction in the perpendicular field components can beachieved with a conductor geometry which consists of pairs of closelyadjacent conductor segments, perpendicular to the band plane, withantiparallel current (I). For each pair of such conductors the magneticfield in the conductor is for the most part parallel to the band plane,which results in a substantially smaller AC power loss (P). The AC powerloss (P) per unit conductor length (l) is given by:

    P/l=4·j.sub.c · -A(x.sub.ec)·F+(∫) A(x) df!,

A(x) being the vector potential at maximum current, x_(ec) being theso-called electrical center of the superconducting band, at which theelectric field is always =0, and F being the cross-sectional area of theband. The integral extends over the entire conductor cross section F. Itcan be seen from the above formula that the current return concept iseffective if the conductor separation (Δ) of the superconductors (3, 3')perpendicular to the band plane is substantially less than the meanderpath width (b). For Δ>>b, the superconductors (3, 3') behave as 2individual conductors with high AC power loss (P). For b=2 mm and d_(SC)=0.5 mm, with current return a reduction in the AC power loss (P) by afactor of 2 can be achieved. The current return design can be realizedwith bands arranged in meander or spiral form, cf. FIG. 3. In this casethe conductor separation (Δ) is preferably selected to be <10 mm.

EXAMPLE 1

With Layer Structure According To FIG. 2

    ______________________________________                                        Rated power P.sub.R     20 kW,                                                Rated voltage U.sub.R   200 V,                                                Rated current I.sub.R   100 A,                                                Maximum current I.sub.max                                                                             300 A,                                                Critical current density j.sub.c                                                                      1 kA/cm.sup.2,                                        Conductor width b       1.4 cm,                                               Meander gap width (9)   1 mm,                                                 Conductor length 1 per module (5)                                                                     126 cm,                                               Total conductor length  8.8 m,                                                Number of modules (5)   7,                                                    AC power loss P at 77K  0.62 W.                                               ______________________________________                                    

A modularly designed high temperature superconductor (3, 3') based onBi:Sr:Ca:Cu=2:2:1:2 was applied to a thickness (d_(SC)) of 1 mm ontoboth sides of a ceramic sheet (1) with an area of 10 cm·10.4 cm and athickness (d₁) of 1 mm. There was a silver layer (2) with a thickness of2 μm between the ceramic sheet (1) and the superconductor (3, 3'). Thesilver acts both as an electrical stabilizer (bypass resistor) and as achemical insulator between the superconductor (3, 3') and the ceramicsubstrate (1). A lead layer (4, 4') with a thickness (d_(NC)) of 10 μm,which likewise contributes to the electrical stabilization, was appliedto the other side of the superconductor (3, 3').

Meander gaps (9) according to FIG. 3 were cut from the superconductinglayer on both sides of the sheet. The two conductor tracks (3, 3') onboth sides of the ceramic sheet (1) are electrically connected to eachother in such a way that the current (I) in directly opposite band partsflows in antiparallel directions. The current return effect for reducingthe AC power loss (P) is thereby achieved.

EXAMPLE 2

Switch Function According To FIG. 1

By enclosing the current limiter modules (5) according to Example 1 witha magnetic field coil (8), as represented in FIG. 1, the deviceaccording to the invention can be used as an active switching element.When the magnetic field is turned on, the critical current strength(j_(c)) in the superconductor (3, 3') is reduced, so that thesuperconductor (3, 3') undergoes transition into the resistive state.This causes the current (I) to be reduced to a fraction of the ratedcurrent (I_(R). Because of the texturing of the superconductor (3, 3'),the reduction in the critical current strength (j_(c)) is a maximum ifthe applied magnetic field is perpendicular to the plane of thesuperconducting band, as in FIG. 1.

Production of a superconducting sheet (1)

Superconductor powder of composition Bi_(a) Sn_(b) Ca_(c) Cu_(d) O_(e)with a, b, d=1.8-2.2, c=0.8-1.2, e=7.5-8.5 is introduced, in the drystate or as a suspension with a liquid, into a suitable flat mold. In apreferred procedure, silver powder and/or Bi₂ O₃ powder is mixed in aconcentration range of 0.5%-5% with this superconductor powder, whichhas a positive influence on the melting and compacting of the melt. Anymaterial which does not react with the powder during the subsequentmelting of the powder and is structurally stable at temperatures ofaround 900° C. is suitable as a casting mold. Molds made of sheetsilver, of nickel alloys with a silver protective layer and ceramicsheets made of magnesium oxide and stabilized zirconium oxide were used.The metal molds were easily able to be provided, for example by deepdrawing or folding, with a rim having a height of approximately 10 mm.Conductive silver was used as the buffer layer (2) or adhesive. Thefilling level was selected in such a way that, when the powder wascompacted to 100% by the melting, a thickness (d_(SC)) of 0.3 mm-3 mmresults. The highest possible so-called green density of the powder,which was achieved by uniaxial subsequent pressing of the loose powderbed, is advantageous for the achievable current density and homogeneity.A pressure of 10 MPa is sufficient for this. The casting method isdescribed in DE-A1-4,234,311.

Application of the Electrical Stabilization

Superconductor sheets (1), which were produced in silver or ceramicmolds, were provided with a metallization (4, 4', 14, 15) which servesas electrical stabilization. For this purpose the silver must be removedfrom the superconductor sheet (1), which can be done before themetallization or after a mechanical stabilization has been applied.

When casting molds of silvered nickel-based alloy are used, separateelectrical stabilization is not necessary if the resistance of thesilver/nickel-based alloy combination already corresponds to that of thesuperconductor (3, 3').

EXAMPLE 3

Square casting molds with dimensions of 100 mm·100 mm, having rims witha height of 6 mm, were folded by hand from a 100 μm thick silver sheet.These casting molds were in each case filled with a slurry of 60 g Bi₂Sr₂ Ca₁ Cu₂ O₈ +δ powder in ethanol, 0≦δ≦0.3. After the liquid haddried, the powder bed was compressed by uniaxial pressing with apressure of 2 GPa. The specimens were then subjected to a heattreatment, in an oxygen atmosphere, which consisted of a melting step at900° C. for a period of time in the range from 20 h-80 h. Homogeneouscompact superconductor sheets (3, 3') with a thickness of approximately1 mm, from which the silver could easily be shaved off, were obtained asa result. A 50 μm thick tin layer was deposited onto thesesuperconductor sheets (3, 3') by flame spraying for electricalstabilization. These superconductor sheets (3, 3') were then adhesivelybonded onto an aluminum sheet and made into a meandering form bywater-jet cutting, conductors with a cross section of 14 mm·1 mm and alength of approximately 70 cm being produced as a result. After thealuminum sheet had been removed, 2 superconductor sheets (3, 3') were ineach case aligned relative to each other in such a way that their trackson the front and rear side of the ceramic sheet (1) run parallel and theends with the silver contacts (10, 11) are on top of each other. Thesilver contacts (10, 11) are bonded on with a silver-filled epoxy resinadhesive, and the ends can be connected in series with low impedanceusing them. The contact resistivity of the adhesive bond withsilver-filled epoxy resin is 0.05 μΩ cm². At a current (I) of 1 kA thecurrent limiter module (5) formed a resistance of 5Ω.

It is expedient to select the ratio of the conductor separation (Δ) tothe meander path width (b) of a current limiter module (5) at <0.5,preferably <0.1.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A current-limiting device, comprising:at leastone superconductor; at least one non-superconducting shunt resistor inparallel with each superconductor, each superconductor having a firstmain superconductor face in contact with a main shunt resistor face of ashunt resistor so as to form a compound conductor generally in the formof a meandering band having a band width greater than approximately 3.5times the superconductor thickness; at least one insulator havingopposing faces each contact with said compound conductor, whereinmultiple compound conductors are arranged with respect to each othersuch that current flows in opposite directions through adjacent compoundconductors, and wherein a superconductor separation of superconductorson opposite sides of an insulator, perpendicular to the plane of themeandering band, is smaller than the band width.
 2. The device of claim1, wherein the band width is greater than approximately 10 times thesuperconductor thickness.
 3. The device of claim 1, wherein:each shuntresistor has an electrical resistance not greater than the electricalresistance of the connected superconductor in a non-superconductingstate of the connected superconductor; and each superconductor has aminimum conductor length l, where

    l>b×(d.sub.SC /ρ.sub.SC +d.sub.NC ρ.sub.NC)×(1.414)×Ur(nI.sub.r),

wherein d_(SC) in the thickness of the superconductor, d_(NC) is thethickness of the shunt resistor, ρ_(SC) and ρ_(NC) are resistivityvalues of the superconductor and shunt resistor, respectively, I_(r) isa rated current and Ur is a rated voltage of a current source, n is aratio of maximum permissible current to I_(r), and b is a strip width ofthe compound conductor.
 4. The device of claim 1, wherein thesuperconductor separation is less than approximately 10 mm.
 5. Thedevice of claim 1, wherein one of the at least one superconductor is incontact with a second insulator.
 6. The device of claim 1, wherein thesuperconductor separation is less than approximately one-half the bandwidth.
 7. The device of claim 6, wherein the superconductor separationis less than approximately one-tenth of the band width.